It could be several problems
- Shielding from the material
- Mechanical load from the casing
- A metal screw next to the antenna
To learn more about the problem, comparing different configurations is the only way to got (As you guessed, it might be shielding, so you could see how the same shape works when printed in white PLA). Since comparing wifi signals is very prone to random changes, it is important to get better information than "random drops". Signal strength is a much better indicator.
A wifi signal test is shown here, comparing a bare PI with one in a aluminum case
Indeed the aluminum case reduces the signal strength, as it was to expect.
For a faster comparison you can use the command 'iwconfig'. It usually shows a signal strength (Tx-Power)
wlp4s0 IEEE 802.11 ESSID:off/any
Mode:Managed Access Point: Not-Associated Tx-Power=20 dBm
Retry short limit:7 RTS thr=2347 B Fragment thr:off
A possible workaround might be to add an external antenna. The raspberry pi actually has solder points to add an antenna connector.
However it might be simpler to produce several iterations of casings and compare the signal strength, depending on availability to SMD soldering.